X# is the name that has been given to a "data-centric" programming language that Microsoft is said to be developing in order to make it easier for a programmer to specify XML-defined data and how it should be handled by the program.

Extensible Forms Description Language (XFDL) is a use of the Extensible Markup Language (XML) that provides a standard way to define the data fields and layout for a complex business or government form for digital storage and display.

XMI (XML Metadata Interchange) is a proposed use of the Extensible Markup Language (XML) that is intended to provide a standard way for programmers and other users to exchange information about metadata (essentially, information about what a set of data consists of and how it is organized).

XMLHttpRequest, also called XHR, is an application program interface (API) that was first used by Microsoft in version 5.0 of its Internet Explorer Web browser to function as an ActiveX object... (Continued)

XPath is a language that describes a way to locate and process items in Extensible Markup Language (XML) documents by using an addressing syntax based on a path through the document's logical structure or hierarchy.